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Nov. 21, 2012 -- Women over age 40 are often urged to get yearly mammograms with the promise that early detection is their best hope for beating breast cancer.

But a new study published in The New England Journal of Medicine suggests that mammograms may not save as many lives as doctors once thought.

The study also finds that the tests may be responsible for substantial harm, causing an estimated 1.3 million women in the U.S. over the last 30 years to be treated for breast cancers that would not have caused clinical symptoms.

Bleyer says the study suggests that while mammograms are good and save lives, they should be used less often.

He supports guidelines from the U.S. Preventive Services Task Force that delay screening for most women until age 50 and only recommend tests every other year until age 75.

Other organizations, including the American Cancer Society and the American College of Obstetricians and Gynecologists, say women should get annual mammograms starting at age 40.

Len Lichtenfeld, MD, deputy chief medical officer at the American Cancer Society, says the study is interesting and will lead to more discussion about mammograms.

“It’s not a study that’s going to close the book, shall we say, on the discussion regarding the value of mammography,” says Lichtenfeld, who was not involved in the research.

Estimating Overdiagnosis of Breast Cancer

The study looked at trends in breast cancer detection in the U.S. from 1976 through 2008.

Researchers compared rates of breast cancer diagnoses before and after mammograms became widely used for screening. They adjusted their results to remove excess cancers thought to be related to the widespread use of hormone replacement therapy (HRT).

Mammograms did help find more early-stage breast cancers. But finding those cancers did not seem to prevent much late-stage disease. The number of late-stage breast cancers, which are most likely to be life-threatening, dropped only slightly, from 102 to 94 cases per 100,000 women.

Researchers think that means that many early breast cancers -- perhaps as many as half of all cancers detected by mammograms -- are probably harmless and are being “overdiagnosed.”

Overdiagnosis is “the diagnosis of nothing, or something that is nothing,” says Bleyer.

“Whatever is found would never have created a problem during that person’s life. They wouldn’t have known about it otherwise,” he says.

The problem is that doctors can’t tell which early cancers may be harmless and which may become life-threatening. So once the call is made, patients are offered treatment including surgery, chemotherapy, and radiation.

“If we had a way to better distinguish within that diagnosis which cancers were likely to cause a problem, we would save a lot of women from unnecessary treatment,” says Sharon Giordano, MD. Giordano is a breast medical oncologist at MD Anderson Cancer Center in Houston. She was not involved in the research.

Does Early Detection Save Lives?

Researchers also looked at whether or not early detection with mammograms was saving lives.

Over the period of the study, death rates from breast cancer in women 40 years or older dropped by 28%, a substantial decline.

But breast cancer death rates dropped even more (42%) among women under age 40, a group that doesn’t routinely get screening mammograms.

“We are left to conclude, as others have, that the good news in breast cancer -- decreasing mortality -- must largely be the result of improved treatment, not screening,” the researchers write.

This isn’t the first study to flag the harms of overdiagnosis tied to mammograms. Ten other studies conducted in Europe and Australia have estimated that somewhere between 10% and 47% of all breast cancers detected in those countries are unnecessarily treated.

Certain cautions apply.

The first is that it’s not possible to directly count cases of overdiagnosis. Studies that try to have to rely on estimates. Those estimates are always subject to errors and biases.

The second caveat is that the study results don’t apply to everyone.

Women at higher risk of cancer because they have a family history or because they carry genes for the disease “really have nothing to do with this,” Bleyer says. “They are in a completely different category and they do need to be detected as early as possible, because they get a worse kind of cancer and they get it earlier in life.”

Women at average risk should carefully weigh the benefits of mammograms against their harms, which include worry, further testing, and sometimes needless treatment.

“We strongly endorse the concept that women and their health professionals have conversations about these issues and come to the conclusion that’s right for them,” Lichtenfeld says.